In general the present invention relates to noise reduction systems and more particularly to method and apparatus for removing background noise from audio signals, especially musical signals.
The presence of background noise, sometimes defined as wholly random signal energy, accompanying audio muscial signals has long been known as an undesirable but apparently unavoidable by-product of the transmission, recording and/or reproduction of audio signals. Many attempts have heretofore been made to reduce the noise content by various frequency filtering schemes. However, noise occurs at all frequencies within the audible band and as such any attempted filtering of the noise usually results in some loss of the musical frequencies.
For example frequency filters designed to remove the "scratch" noise during the reproduction from phonograph records constitutes a simple noise filter. The high frequency "scratch" sounds from the record are filtered out by cutting off the high frequency components of the reproduced signal. Inherently, the "scratch" filter removes the high frequency content of the record from the reproduced signal, diminishing the fidelity of the reproduction.
A more sophisticated filtering system employs what is known as dynamic filtering. Here the amount of rolloff or cutoff of the high frequencies is adjusted by a control voltage which is a function of the energy content of the reproduced or transmitted musical signal. These dynamic filters represent an improvement over the simple "scratch filter;" however, known dynamic filtering techniques only remove the higher frequency noise and cause a certain amount of undesirable modulation of the audible higher frequencies.
Another attempt at noise reduction has been the provision of multiple band dynamic filtering. This uses the dynamic filtering principle as discussed above together with a series or multiplicity of controllable band-pass filters. The individual filters are dynamically operated by the frequency content of the incoming signal. Thus during processing of the signal, the gates individually and collectively open and close allowing the musical content of the signal to pass through while blocking the noise content. Unfortunately, multiple band dynamic filtering systems heretofore developed have exhibited an objectionable audio "swish" sound coincident with each opening of one of the band-pass filters. The "swishing" sounds constitute a disturbing psycho-acoustic effect and have prevented this type of filtering system from gaining wide acceptance in the high fidelity equipment industry.
Also, the "swish" sounds heard during the opening and closing of the band-pass filter gates demonstrates one of the inherent difficulties in reducing or eliminating noise from an otherwise high fidelity audio signal. The noise exists substantially continuously and uniformly within the band-pass of the musical material. The noise may be of greater or lesser magnitude relative to the strength of the audio information signal, however it is always superimposed thereon and as a practical matter inseparable therefrom.
Some systems have recognized the inseparability of the noise and information content of the signal and have merely provided a signal processing system for selectively increasing the amplitude of the musical material. However usually these systems require a preencoding of the musical signal such as by special encoding of frequency modulated signals, phonograph recorded signals and/or tape recorded signals. For example, one system widely used today for tape recording and reproducing systems provides for encoded recording of lower level musical passages at a higher amplitude, above the noise level or noise floor of the recording system. During playback the encoding/decoding process converts the playback signal back down to a correct amplitude for the lower level passages. This processing does provide a reduction of noise, however it can only be used in a "closed" system, i.e., where the source signal has been properly encoded.